The copyleft definition

Copyleft means that a derived work of a copyleft software, that are not used for internal or personal use, must include the source code and released under the same terms of the original work. Copyleft is common in many licences including the GPL, the Lesser General Public License, the QPL, etc.

Many licences are not Copyleft - most notably the BSD and MIT/X11 licences. Software released under such licences can be derived into a proprietary software product by a third party, and often have been.